One of the major problems facing industries such as mining, precious metals, and energy industries (e.g., coal mining and coal-fired power plants) is removal of selenium from process effluent to meet federal and state compliance standards. Stringent standards for the maximum level of pollutants in water to be used for drinking or related to ground water systems are being promulgated by federal and state agencies. The current allowable maximum concentration level for selenium in waste waters set by federal standards is 1.0 milligrams per liter. Selenium removal from ground water presents a challenge in many geographical areas of the United States.
Several methods are available for reducing selenium concentrations to acceptable levels in aqueous solutions. One method employed to remove or substantially reduce the concentration of soluble inorganic pollutants such as heavy metals in water is chemical precipitation of the metals as their oxides or their hydroxides. This precipitation generally is effected by the addition of lime, alum or an iron salt to the water at an appropriate pH. Other treatment methods, such as ion exchange, reverse osmosis, electrolysis or distillation, also can be effective in removing various pollutants. However, these methods are considerably more expensive and generally narrower in application scope than is desirable for the treatment of water containing higher levels of selenium. Often it is difficult to remove selenium using conventional methods.
It is known that selenium ions can be removed from aqueous systems employing chemical precipitation if the selenium is present in the selenite (SeO.sub.3.sup.-2) form. Generally, such precipitation methods comprise treating the selenium-containing aqueous system with an iron salt, such as ferric or ferrous sulfate, chloride or hydroxide, or with aluminum or zinc in some appropriate form such as powder or granules. However, such chemical precipitation methods provide only very limited removal of selenium when it is present in the selenate (SeO.sub.4.sup.-2) form. Therefore, when selenium is present in the selenate oxidation state (Se.sup.+6), its removal generally is effected by either ion exchange or reverse osmosis.
In particular, experimental studies have shown that chemical precipitation employing ferric sulfate can achieve a significant removal of selenium in the selenite oxidation state (Se.sup.+4) from an aqueous stream containing low levels of selenite. More particularly, when ground water containing 0.03 milligrams per liter of selenium in the selenite oxidation state and having pH of 5.5 is treated with 30 milligrams of ferric sulfate per liter, about 85% of the selenium is removed from the water (U.S. Environmental Protection Agency, "Manual of Treatment Techniques for Meeting the Interim Primary Drinking Water Regulations," May 1977, pages 29-31).
U.S. Pat. No. 3,933,635 discloses a process for removing selenium ions present in the selenite oxidation state from acidic process waters. Acidic process water, having a pH of about 1.0 to 4.0, is reacted with a metallic reducing agent at a temperature in the range of from about 250.degree. C. to about 85.degree. C. for a sufficient time to reduce the soluble selenium in the selenite oxidation state to insoluble elemental selenium. Preferably, the temperature is maintained in the range of from about 50.degree. C. to about 70.degree. C. The reducing agent can comprise aluminum, iron or zinc in an appropriate form, such as, for example, a powder, scrap fragments, granules, wools, and the like. The preferred reducing agent for selenium in the selenite oxidation state is zinc powder.
In contrast, laboratory tests and pilot plant studies have shown that chemical precipitation, employing alum, lime, ferrous sulfate or ferric sulfate, is substantially ineffective for removing selenium in the selenate oxidation state from water. Studies on water having a selenium concentration of 0.03 to 10 milligrams per liter have shown that the conventional chemical precipitation methods remove less than 10 percent of the selenium from the water (U.S. Environmental Protection Agency, "Manual of Treatment Techniques for Meeting the Interim Primary Drinking Water Regulations," May 1977, pages 29-31).
It has been shown that selenium ions in the selenate oxidation state can be removed by ion exchange or reverse osmosis. As previously indicated, these methods are prohibitively expensive when large volumes of an aqueous solution must be treated. Further, both methods produce a contaminated regeneration effluent that requires further treatment for selenium fixation or removal before disposal. Thus, the selenium removal problem still exists but in a more highly concentrated solution.
U.S. Pat. No. 4,405,464 provides a method by which selenium, as the selenate, can be chemically precipitated from an aqueous system using metallic iron. This process is also disclosed as being capable of removing a substantial portion of any selenium in its selenite oxidation state. This process is economically more attractive than either the ion exchange or reverse osmosis methods, which have been proposed or which are currently in use. However, this method is not suitable for aqueous solutions having a pH greater than 6.0. Thus, if the water is alkaline or neutral, it is preferably acidified through the addition of a quantity of a mineral acid such as, for example, hydrochloric acid or sulfuric acid or any other acidic solution such as acidic mill process water.
U.S. Pat. No. 5,603,838 provides a method by which selenium and arsenic are chemically precipitated from an aqueous system using metallic iron. This process is also disclosed as being capable of removing a substantial portion of any selenium in its selenite and/or selenate oxidation state. This process uses an lanthanum oxide/alumina combination to form an insoluble selenium complex, which is subsequently separated from the aqueous solution to remove the selenium. Although the disclosed process removes both selenite and selenate states of selenium, the disclosed process requires relatively expensive reactants to remove selenium from a waste stream.
Literature published to date by the EPA and others reports removal efficiency of greater than 98% of selenium with ALCOA.TM. F-1 alumina. However, the high chemical impurity level in F-I results in a low point of zero charge, which means low specific adsorption of negatively charged ions. F-1 alumina also has an inherently low chemisorption capacity, which makes this material a poor choice as an adsorbent for selenium in any form.
What is needed in the art is a consistent, low cost, efficient method of removing selenium from a waste stream. The process should be capable of removing selenium from a solid or liquid waste stream. Further, the process should effectively remove both the selenite and selenate states of selenium without the use of expensive reactants.